As shown in FIG. 1, a spacecraft 10 includes a number of rigid solar panels 12, which are shown in their deployed position. Solar arrays 14 which may include hundreds or thousands of solar cells 16 bonded to each solar panel 12 are used to provide electrical power to drive a variety of spacecraft systems and to recharge its batteries. The spacecraft rotates the solar panels 12 so that they receive direct illumination from the sun 18 to increase efficiency.
The solar cells 16 include a flat photovoltaic wafer made from n-type or p-type crystalline semiconductor material, such as silicon, gallium-arsenide or germanium in or on which a thin surface layer of the opposite conductivity type is formed. The interface between the surface layer and the main region of the wafer defines a semiconductor junction. Illumination of the thin surface layer causes liberation of charge carriers, including electrons and holes in the region of the semiconductor junction, which migrate toward opposite surfaces to establish a potential across the solar cell.
The solar panel 12 has three primary functions. First, the panel provides a rigid support structure with sufficient axial and bending stiffness for carrying the solar cell array 14 through a dynamically active launch environment into orbit and positioning it to receive illumination. Secondly, the front surface of the solar panel 12 to which the cells are bonded is electrically inert so that the individual solar cells 16 are electrically isolated. Lastly, the solar panel 12 serves as a heat sink to the space facing side (opposite sun 18) for the solar cell array. The spacecraft 10 of FIG. 1 is a rigid array as described. Thin film solar arrays are also possible that utilize a different stowed structure during launch that is lighter and more load efficient than the rigid panels, which might include a gossamer deployment structure that holds the solar cells out in space on the thin film solar array.
There are a number of important issues associated with solar panel design regardless of being a rigid array, a thin film array, or a flexible array. The heat sink capabilities of the substrate must be sufficient to cool the solar cell array to maintain power efficiency. The solar panel should have low and/or matched thermal expansion properties compared to the solar cells. The temperature on the illuminated side of the array can be as high as +70° C. and can be as low as −180° C. or lower for thin film arrays on the back surface, which faces deep space. Due to these thermal expansion properties, warping or damage of the solar panel can occur.
A paramount concern in solar panel design is weight. Existing spacecraft can have eight solar panels, four per side, where the structural constituents weigh approximately 10 lbs (4.5 kg) per panel. Currently, the cost of flying a spacecraft can be estimated as high as $25,000 or more per pound over the lifetime of the spacecraft. Hence, the weight of the solar panel impacts the overall cost of operating a spacecraft.
Solar cells are often supported by a graphite facesheet and an insulating layer facesheet that are bonded to the solar cell by a silicone-based adhesive called “RTV” adhesive. The cross-section of such solar cells, from top to bottom, typically include a solar cell CIC (coverglass, interconnects, cell), a layer of the RTV adhesive, an insulative layer facesheet (e.g., a DuPont™ Tedlar® polyvinyl fluoride (PVF) film or a polyimide such as a Dupont™ Kapton® polyimide film), and a graphite or Kevlar® facesheet on a honeycomb substrate. The RTV is acknowledged in the industry as a source of degradation over the life of the solar arrays. Attempts to reduce the degradation of the solar arrays include the development of ultra low outgassing RTVs.
RTVs have a coefficient of thermal expansion (“CTE”) in the range of 100-200 ppm/° C. A Kapton® polyimide film has a CTE around 17-20 ppm/° C. and some solar cells have CTEs around 3-8 ppm/° C. The mismatch in the CTEs between the solar cell, RTV adhesive, and Kapton® polyimide film is inherent in using this accepted adhesive as the standard adhesive for constructing solar arrays. One way to combat this mismatch has been to optimize the thickness of the RTV. But, increasing the thickness adds weight to the solar array, which depending upon the application intended for the solar array may be undesirable.